How to create volume scattering material not throwing shadow?
Volume scatter will always make your lights seem darker. So you can make the lights brighter until you get the level you want.
You can animate the density.
$0$ density will make the volume scatter transparent, then bring up the value when you need the volumetric lighting.
As an alternative don't use volume scattering in the world, but on an object with volume scatter as voume, in a way in which you get the rays as you want, but not on the rest of the scene.
Another alternative is to disable ray visibility on light so that is invisible to Volume Scatter.